We have improved the techniques for monitoring parameters of myoblast growth and differentiation using phase contrast microscopic photography. Individual groups of cells are monitored reducing variation from plate to plate. Myoblasts were examined to determine if amplified replication occurs during the critical period of cellular differentiation. Density analysis of DNA replicated during differentiation indicates no detectable amplified DNA replication (less than 10 to the minus 5th power). Also, conversion of a portion of the genome into an episome (nonchromosomal state) was examined. No evidence of episomes smaller than 30S was detected. Thus, neither DNA amplification nor episome formation appear to be major events in myoblast differentiation. On the other hand, we have shown that if the myoblasts are triggered to differentiate (shift in medium) but blocked from cytodifferentiation by ethidium bromide, the myoblasts become postmitotic independent of cytodifferentiation. This unusual finding suggests that during the triggering for differentiation, the chromatin may be reorganized extensively resulting in a postmitotic state independent of cytodifferentiation. We have also completed a study of BrdUrd inhibition in eucaryotic cells compared with procaryotic cells. Apparently Brdurd inhibits differential gene expression only in eucaryotic cells suggesting that chromatin proteins may be required for BrdUrd inhibition. BIBLIOGRAPHIC REFERENCES: Brunk, C.F. and Yaffe, D. The Reversible Inhibition of Myoblast Fusion by Ethidium Bromide (EB). Exp. Cell Res. 99: 310-318, 1976. Yee, W.S., Decker, R.W., Brunk, C.F. Incorporation of Tritium-Labeled Thymidine Monophosphate into Nuclear DNA by Permeabilized Yeast Cells. Biochim. Biophys. Acta 447: 385-390, 1976.